Rapid detection of microbial pathogens in body tissues is a challenging task. Prompt diagnosis is crucial for successful therapeutic intervention in the case of infection by fast-growing pathogens or with immune-compromised individuals. The problem is aggravated due to increasing use of immunosuppressive agents in patients who receive transplants, implants, or more intensive anticancer therapies. Conventionally, detecting and monitoring infectious processes are achieved by use of various non-invasive imaging techniques, including conventional radiography, ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI). However, these techniques rely solely on morphological changes and, therefore, most abnormalities can only be detected at advanced stages of disease and diagnosis of early infections and differentiation between active and structural but indolent alterations following surgery or other interventions can be difficult. Also, morphological imaging methods cannot differentiate between sterile inflammation and infection and do not allow for timely monitoring to access the success of antimicrobial therapy at an early stage.
Fluorescence Molecular Tomography (FMT) is a powerful approach to image microbial infections in body tissues. It is based on using fluorescently labeled compounds (e.g. antibiotics) that recognize pathogen-specific targets, thereby rendering the pathogen cells fluorescent after binding to the targets. In FMT, imaging of fluorescent material (labeled microbial pathogens) in body is achieved through excitation by powerful light source and detection of the induced fluorescence. Moving the excitation source around a scanned object with simultaneous signal acquisition allows 3-D imaging of the area of interest. For these applications fluorescent group attached to the “address” molecule must have excitation and emission maxima in near-infrared (NIR) spectral range where body tissues are transparent.
The near infrared (NIR) range of the spectrum is a very promising area for fluorescence detection and imaging. Most fluorophores operate in the visible to ultraviolet range of the spectrum. In NIR much less background/noise is observed in both in vitro assays and tissue samples, thereby increasing detection sensitivity. Notably, body tissues have a transparency window in the region 650-800 nm with a maximal transparency at 680 nm, which enables non-invasive imaging of NIR dyes in the living body. Therefore, long wavelength emitting dyes are of a great demand for numerous biomedical applications such as imaging of pathogens inside the body. However, conventional NIR dyes tend to be large in size and possess groups with a stable electric charge (introduced to enhance a solubility of the compounds), and when conjugated to a targeting agent such as a drug, may affect the interaction of the modified drug with a target and cell permeability of the labeled drug derivatives. In particular, most current NIR fluorescent labels are bulky and contain a stable positive charge, which is likely to disable the “address” molecules by affecting their affinity to the targets. Therefore, new NIR labels, especially of small molecules, with desired properties are still in high demand.